Insulated pipes are ordinarily used for transporting a flowing heating or cooling fluid, the medium, as in district-heating or district-cooling piping, and have a steel pipe for transporting the flowing fluid, the medium. The steel pipe is then embedded in insulation, usually made of polyurethane (PUR) foam, and on the outside there is an outer casing that protects from moisture, an outer pipe, made of a weldable plastic material, commonly of polyethylene and so-called HDPE. The piping is fabricated in a factory in sections, with an outer pipe, an inner pipe, and insulation, which are then transported out to the installation site. Insulated piping of this type can also be used for the transporting of oil and is then commonly called a “pipeline”.
The piping is intended to be concealed underground, which entails large stresses. There will be a mechanical impact on the pipes owing to temperature changes, which result in expansion and contraction of the material. The piping is also exposed to water or moisture that can penetrate it and cause the steel pipe to corrode, which over time can result in leaking and consequently require repairs. The piping often runs under walls and the like, making it difficult and costly, in the event of leaking, to gain access to the damaged piping that has to be dug out.
Even though in certain district-heating piping there is, in the insulating foam between the steel pipe and outer pipe, an alarm wire, by means of which a leak can be located with relatively high accuracy, the cost of repairs will be high. Consequently, it is of greatest importance to produce installations—joints—of high quality and strength, both in new installation and when making repairs. Inconsistent quality, or, in the worst case, systematic defects in assembly, can in the long run be devastating to the overall economics of a district-heating system.
There are presently on the market a number of methods of joining the moisture-barrier outer pipe of district-heating piping. Experience has shown that methods that employ welded joints often have advantages over the long term, compared to other solutions. However, welding is not totally without problems.
The power supply is a major problem. There can easily be heavy power losses, because current transformation can be necessary and the current has to be conducted over relatively long distances to reach the welding site itself. The available power supplies are also not the best.
An example of prior art is EP 1266745, which presents a method and apparatus for electric-resistance welding of plastic pipes. The apparatus has a power unit for supplying current to a welding sleeve coupling. The document presents a welded sleeve coupling having two electrical contacts that make contact with two plastic-pipe ends. The contacts are connected to a power controller and to an alternating-current source. The power unit comprises a converter, transformer, rectifier, bypass circuit and filter circuit. The power unit operates at an operating frequency in the range of from 150 to 300 kHz. With these frequency levels, the power unit's output side has to be furnished with a rectifier and filtering circuits, resulting in heavy weight and major problems with efficiency and EMI radiation. In association with rectification and filtering on the output side, energy losses occur—through heating, for example—which has an adverse impact on efficiency. This is especially hard to overcome at higher powers.